1. Field of the Invention
The field of the present invention relates to overhead valve engines, such as overhead cam engines, for use in a variety of applications, such as walk behind lawnmowers, lawn and garden implements, or in small utility vehicles such as riding lawnmowers, lawn tractors, and the like. In particular, the invention relates a drive train for such engines in which the crankshaft drives an internally profiled cam gear which in turn actuates a pair of rocker arms having cam followers in engagement with the cam profile to open and close intake and exhaust valves in the cylinder head.
2. Description of the Related Art
Prior known engines containing drive trains of an overhead valve design are well known in the art. For example, in one known arrangement, a crankshaft drives a camshaft located near the crankcase through a gear set. The camshaft includes one or more lobes which actuate a pair of cam followers mounted for rotation on a cam follower shaft. The cam followers in turn actuate push rods extending from the crankcase to the cylinder head, and the push rods rotate a pair of rocker arms mounted in the cylinder head to open and close the intake and exhaust valves.
In another known arrangement, a camshaft located in the cylinder head is driven from the crankshaft by means of a belt, chain, or the like. The camshaft includes one or more lobes that actuate the intake and exhaust valves either directly, or through a pair of rocker arms rotatably mounted in the cylinder head.
A disadvantage with the first arrangement is that the several components of the drive train, including the camshaft, camshaft lobes, cam follower shaft, cam followers, push rods, and rocker arms tends to increase the overall size of the engine. The multiple components also increase the cost and complexity of the engine, the difficulty of assembly, and the likelihood of failure of one of the components.
A disadvantage of the second arrangement is that locating the camshaft in the cylinder head increases the width of the cylinder head due to the lateral space between cam lobes and/or between a cam lobe and the pulley or sprocket which is mounted on the camshaft and driven from the crankshaft. The location of the camshaft directly above the valves, when the camshaft actuates the valves directly, also increases the length of the cylinder head. In addition, the length of the cylinder head is further increased to accommodate the relatively large pulley or sprocket mounted on the camshaft which is necessary for speed reduction. Further, the belt, chain or the like which drives the camshaft from the crankshaft is prone to wearing or breakage.
What is needed is a drive train for an engine which is compact, such that the drive train may allow a smaller engine height and width.
A further need is for a drive train for an engine where the drive train is simplified and includes a minimum of components.